1. Field of the Invention
The present invention generally relates to an optical semiconductor device that is applied to optical fiber communications, and specifically relates to an array of optical semiconductor light receiving elements as light receiving elements (photodiodes: PDs) that can adapt to multi-channelization.
2. The Related Technology
With the recent development of an optical fiber communication technology such as a multiple wavelength communication, a light receiving element that can detect light having more channels is demanded. Meanwhile, in order to prevent increase of size of a device with multi-channelization, downsizing and integration of a device is also demanded. To fulfill these demands, an optical semiconductor device on which light receiving elements in an array are formed is widely used since it can receive light of multi-channel and is compact.
FIG. 1A is an external view of a conventional optical semiconductor device described in Japanese patent Laid-Open No. 2007-266251. FIG. 1B is a cross-sectional view of the conventional optical semiconductor device, the cross-sectional view including a light receiving section. FIG. 1A illustrates, as an example, an array of optical semiconductor elements, each having a light receiving section, the array being composed of four elements. The number of the elements can be increased or decreased according to application.
The optical semiconductor device in FIGS. 1A and 1B has a light absorbing layer 112 formed on a conductive semiconductor substrate 110, and a plurality of diffusion regions 120 that has a conductive property opposite to that of the conductive semiconductor substrate 110. The light absorbing layer 112 has an insulation property. In such a configuration, immediately on the light absorbing layer 112, a conductive semiconductor layer 114 is disposed, and the diffusion regions 120 are formed in the conductive semiconductor layer 114. On the semiconductor substrate 110, a back surface electrode 118 is formed by, for example, evaporation, and on the conductive semiconductor layer 114, an insulating film 116 and a front surface electrode 119 are formed. In this device, an optical semiconductor element 100 is mounted in such a way that the back surface electrode 118 is fixed with the use of metal solder 130, and the front surface electrode 119 is connected to an electrical wiring 136 formed on an electrical wiring board 134 with the use of a bonding wire 132.
As a material of the optical semiconductor element 100, silicon (Si), germanium (Ge), gallium arsenide (GaAs), indium phosphide (InP) or the like is used. Hereinafter, the optical semiconductor element using an InP-based material, which is widely used for long-distance optical fiber communications, will be described.
The conductive semiconductor substrate 110 is made of n-type InP (carrier concentration: 1×1018 cm−3), the light absorbing layer 112 is made of insulating (n-type) indium gallium arsenide (InGaAs, carrier concentration: 1×1014 cm−3), the conductive semiconductor layer 114 is made of n-type InP (carrier concentration: 1×1017 cm−3), and the diffusion regions 120 formed in the conductive semiconductor layer 114 are made of Zn-doped p-type InP (carrier concentration: 1×1018 cm−3). For the insulating film 116 formed on the conductive semiconductor layer 114, silicon nitride (SiN) is used. The insulating film 116 has a passivation function for semiconductor junction, and also serves as an anti-reflective coating when light enters.
A light receiving diameter of a light receiving section 140 is 80 μm; an interval between light receiving elements is 250 μm; and a thickness of the conductive semiconductor substrate 110 is about 200 μm.
In order that the back surface electrode 118 effectively functions as a common cathode of the array of light receiving elements, an ohmic electrode is commonly disposed. That is, an alloy is inserted for reducing a Schottky barrier at an interface between the InP substrate 110 and the metal solder 130. Since this conventional example uses an n-type substrate, an alloy of germanium containing gold and nickel is used. The alloy is deposited on the InP substrate by evaporation, and after that gold and germanium are diffused into InP by heat treatment, thereby reducing the Schottky barrier and making the interface ohmic. Although not illustrated in FIG. 1, on the bottom of the ohmic electrode 118, an electrode that contains titanium, platinum, gold or the like may be further added.
Operation of the optical semiconductor device illustrated in FIGS. 1A and 1B will be described. First, a reverse bias voltage is applied between the front surface electrode 119 and the back surface electrode 118. As illustrated in FIG. 1B, most of incident light 150 inputted into the light receiving section 140 thorough the insulating film 116 from the surface is photoelectrically converted into both carriers of electrons 171 and holes 172 in the light absorbing layer 112. In the light absorbing layer 112 (insulating InGaAs) that was depleted by the reverse bias voltage, a gradient of an energy band occurs. Accordingly, each of the carriers, electrons 171 and holes 172 generated in the light absorbing layer 112 move by drift to the semiconductor substrate 110 (n-type InP) and the p-type diffusion regions 120 (p-type InP), respectively, and are finally emitted outside from the electrodes formed on the front and back surfaces.
Part of the incident light 150 inputted to the light absorbing layer 112 is not completely photoelectrically converted in the light absorbing layer 112, and becomes a substrate-transmitting light 152. The substrate-transmitting light 152 is reflected by the back surface electrode 118 and part of the reflected light may be inputted into the light absorbing layer 112 again, but some of the light reaches an adjacent element 162, as indicated by a dashed line arrow 154 in FIG. 1B.